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<h1>Circle Cutter</h1>

<p>The circle cutter is based on a standard marking gauge.</p>

<p class="imgbox"><img src="fig-001.png" /><br>Figure 1: Schemactic Drawing</p>

<p>The existing adjustment block gets another hole for the center pin.</p>

<p>A steel block is mounted under the end side with 2 cylinder screws. A hole in the bottom takes a steel cylinder knife, which is fixed from the side with an adjustement screw.</p>
<p>If the measurements <i>a</i> and <i>b</i> are identical, the cutting radius can be directly read from the scale on the marking gauge. But that depends much on the shape and position of the cutting edge. For example, if you turn the knife to cut the inner radius, the radius will likely be different.</p>
<p>It is advisable to measure b from the center of the knife hole. Then the cutting offset is determined by two factors:</p>

<ol>
<li>the differnce in radius between drill hole and knife cylinder</li>
<li>the distance of the cutting edge to the center of the knife cylinder.</li>
</ol>

<p class="imgbox"><img src="fig-002.png " /><br/>Cutting Line Offset</p>

<p>The knife block's front edge must be aligned with the front edge of the marking gauge.</p>

<p>When a cylinder shapedsteel is used, the grindingmust be made so thatthe cutting edge is vertical.This might bedifficultwith a cylinder. So a better choice might be a square blade.</p>

<p>An advanced shape might also be possible: One half is ground off (right side). Then we have a perfect vertical edge. Then the left half is flattend, which makes a plane that pushes the wood fibers gently away to the left. Another bevel on the bottom left side reduces the pushing effect for the deeper layers that might not have completely loosened.</p>

<p class="imgbox"><img src="fig-003.png " /><br/>Advanced Cutting Line</p>

<p>Using a square blade opens up a lot of other options: Simple mounting</p>

<p>Alternative knife holder: </p>

<p>Cuboid metal  blade guide block (4). Two tapped drill holes (4a, 4b) in the top are used to attach the block to the marking gauge (1) with screws (2a, 2b) and washers (3a, 3b). The holes (1a, 1b) in the marking gage should be a little bit larger than the drill holes (4a, 4b) in order to allow fine adjustments such as turning and shifting the blade guide block (4) a little bit.</p>

<p>Two adjustable metal plates (5a, 5b), mounted on the front side, act as guides for the blade (12), and ensure that the blade is held vertical. Thickness should be about the thickness of the blade, but that is not so important. They are fixed with two screws (6a, 6b) and washers (7a, 7b).</p>

<p>One blade clamping block (8), attached to the front side with 2 mounting screws on the sides (9a, 9b) and washers (11a, 11b). The screws reach into tapped holes (4c, 4d) in the blade guide block (4). Two vertical arranged tapped through-holes (9a, 9b) allow for two socket screws (12a, 12b) to fix the blade position against the blade guide block.</p>

<p>4 blind holes provide space for the guide plate mounting screw heads.</p>

<p>Parts</p>

<ol>
<li>marking gauge (stock tool)</li>
<li>2 machine screws M4x15</li>
<li>2 washers ~8x4.5x1.5 mm</li>
<li>blade guide block, brass, ~60x40x30 mm, cut from stock material.</li>
<li>2 guide plates, steel, ~30x30x2 mm</li>
<li>4 guide plate mounting screws, M3x10.</li>
<li>4 washers ~6x3.5x1.5 mm</li>
<li>blade clamping block, brass, 60x30x10 mm.</li>
<li>2 clamping block mounting screws, M4x20 mm</li>
<li>2 washers ~8x4.5x1.5 mm</li>
<li>2 socket screws M3x10 mm</li>
<li>steel blade, ~50x8x3 mm, with vertical cutting edge.</li>
</ol>


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